Ultrasound ranging is a technique for computing the distance between two ultrasound transducers. The principle of ultrasound ranging is illustrated in FIG. 1, which shows two ultrasound transducers 10,20 separated by a distance. One of the ultrasound transducers is designated as a transmitting transducer 10 and the other is designated as a receiving transducer 20. To measure the distance between the transducers 10,20, the transmitting transducer 10 transmits an ultrasound pulse 25, which is detected by the receiving transducer 20. The distance, d, between the transducers 10,20 is computed asd=vτwhere v is the velocity of the ultrasound pulse 25 in the medium between the transducers 10,20 and τ is the time of flight of the ultrasound pulse 25 in traveling from the transmitting transducer 10 to the receiving transducer 20.
One application of ultrasound ranging is in ultrasound positional tracking to track the position of a device within a three-dimensional (3-D) coordinate system. Referring to FIG. 2, this is accomplished by mounting one or more ranging transducers 110 on the device 115 being tracked and providing four or more reference transducers 120-1 to 1204 that are spaced apart. In this particular example, the device 115 being tracked is a catheter tip. The ranging transducer 110 acts as a receiving transducer and each of the reference transducers 120-1 to 120-4 can act both as a receiving and transmitting transducer.
To establish the 3-D coordinate system, the reference transducers 120-1 to 120-4 are sequentially excited to transmit ultrasound pulses (not shown). When each reference transducer 120-1 to 120-4 transmits an ultrasound pulse, the other reference transducers 120-1 to 120-4 detect the ultrasound pulse. The relative distances between the reference transducers 120-1 to 120-4 are then computed by performing ultrasound ranging on each of the detected ultrasound pulses. The computed distances are then triangulated to determine the relative positions between the reference transducers 120-1 to 120-4 in 3-D space. The relative positions between the reference transducers 120-1 to 120-4 are then mapped onto the 3-D coordinate system to provide a reference for tracking the position of the ranging transducer 110 in the 3-D coordinate system.
To track the position of the ranging transducer 110, and hence the device 115 carrying the ranging transducer 110, in the 3-D coordinate system, the reference transducers 120-1 to 120-4 are sequentially excited to transmit ultrasound pulses. When each of the reference transducers 120-1 to 120-4 transmits an ultrasound pulse, the ranging transducer 110 detects the ultrasound pulse. The distance d1-d4 between the ranging transducer 110 and each of the reference transducers 120-1 to 120-4 is computed by performing ultrasound ranging on each of the detected ultrasound pulses. The computed distances are then triangulated to determine the relative position of the ranging transducer 110 to the reference transducers 120-1 to 120-4 in 3-D space. The position of the ranging transducer 110 in the 3-D coordinate system is then determined based on the relative position of the ranging transducer 110 to the reference transducers 120-1 to 120-4 and the known positions of reference transducers 120-1 to 1204 in the 3-D coordinate system.
An example of a tracking system using ultrasound ranging is the Realtime Position Management™ (RPM) tracking system developed commercially by Cardiac Pathways Corporation, now part of Boston Scientific Corp. The RPM system uses ultrasound ranging to track the positions of medical devices, including reference catheters, mapping catheters and ablation catheters.
Because ultrasound ranging relies on the transmission and detection of ultrasound pulses to measure distance, it is vulnerable to ultrasound interference from ultrasound sources, e.g., an ultrasound imager. For example, ultrasound interference may be detected by the receiving transducer 20 and misinterpreted as an ultrasound pulse from the transmitting transducer 10, producing an erroneous distance measurement.
Therefore, there is need for systems and methods that enable the use of ultrasound ranging in the presence of ultrasound interference.